KUALA LUMPUR, Malaysia — Dozens of ships and aircraft have failed to find any piece of the missing Boeing 777 jet that vanished more than two days ago above waters south of Vietnam as investigators pursued “every angle” to explain its disappearance, including hijacking, Malaysia’s civil aviation chief said Monday.

Malaysian maritime officials found some oil slicks in the South China Sea and sent a sample to a lab to see if it came from the plane, the Department of Civil Aviation chief, Azharuddin Abdul Rahman, told a news conference.

Additional Photos

A U.S. Navy helicopter lands aboard Destroyer USS Pinckney during a crew swap before returning to a search and rescue mission for the missing Malaysian airlines flight MH370 in the Gulf of Thailand on Sunday. Scores of ships and aircraft from several countries continued to hunt for the plane and the 227 passengers and 12 crew members on board. The Associated Press/Navy Media Content Service, Senior Chief Petty Officer Chris D. Boardman

Italian Luigi Maraldi, left, whose stolen passport was used by a passenger boarding a missing Malaysian airliner, shows his passport as he reports himself to Thai police at Phuket police station in Phuket province, southern Thailand on Sunday. Maraldi spoke at a police news conference where he showed his current passport, which replaced the stolen one, and expressed surprise that anyone could use his old one. The Associated Press

Hundreds of distraught relatives were gathered in a hotel in Beijing, waiting to be flown to Malaysia. Of the 227 passengers, two-thirds were Chinese. There were also 38 passengers and 12 crew members from Malaysia, and others from elsewhere in Asia, Europe and North America, including three Americans.

“We accept God’s will. Whether he is found alive or dead, we surrender to Allah,” said Selamat Omar, a Malaysian whose 29-year-old son Mohamad Khairul Amri Selamat was heading to Beijing for a business trip. He said he was expecting a call from his son after the flight’s scheduled arrival time at 6:30 a.m. Saturday. Instead he got a call from the airline to say the plane was missing.

Vietnamese ships working throughout the night could not find a rectangular object spotted Sunday afternoon that was thought to be one of the doors of a Malaysia Airlines passenger jet.

“We have not found anything that appears to be objects from the aircraft,” Azharuddin said, adding that the search operation has involved 34 aircraft and 40 ships covering a 50-nautical mile radius from the point the plane vanished from radar screens early Saturday about one hour into a flight from Kuala Lumpur to Beijing.

He said officials from the U.S. Federal Aviation Administration and the U.S. National Safety Transportation Board have arrived to help in the investigation.

As hope faded for relatives of the 239 people who were aboard Flight MH370, attention focused on how two passengers managed to board the ill-fated aircraft using stolen passports. Interpol confirmed it knew about the stolen passports but said no authorities checked its vast databases on stolen documents before the jet departed

Warning that “only a handful of countries” routinely make such checks, Interpol secretary general Ronald Noble chided authorities for “waiting for a tragedy to put prudent security measures in place at borders and boarding gates.”

Still, there was no indication that the two men had anything to do with the tragedy.

Possible causes of the apparent crash include an explosion, catastrophic engine failure, extreme turbulence, or pilot error or even suicide.

Azharuddin acknowledged many theories about the plane’s disappearance, including hijacking.

“We are not discounting this. We are looking at every angle but again, we have to find concrete evidence,” he said.

The baggage of five passengers who had checked in to the flight but did not board the plane were removed before it departed, he said. Airport security was strict according to international standards, surveillance has been done and the airport has been audited, he said.

On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight’s manifest matched the names on two passports reported stolen in Thailand.

“I can confirm that we have the visuals of these two people on CCTV,” Malaysian Transport Minister Hishammuddin Hussein said at a news conference late Sunday, adding that the footage was being examined. “We have intelligence agencies, both local and international, on board.”

The thefts of the two passports — one belonging to Austrian Christian Kozel and the other to Luigi Maraldi of Italy — were entered into Interpol’s database after they were stolen in Thailand in 2012 and last year, the police body said.

Electronic booking records show that one-way tickets with those names were issued Thursday from a travel agency in the beach resort of Pattaya in eastern Thailand. A person who answered the phone at the agency said she could not comment.

But no authorities in Malaysia or elsewhere checked the passports against the database of 40 million stolen or lost travel documents before the Malaysia Airlines plane took off.

A telephone operator on a China-based KLM hotline confirmed Sunday that passengers named Maraldi and Kozel had been booked on one-way tickets on the same KLM flight, flying from Beijing to Amsterdam on Saturday. Maraldi was to fly on to Copenhagen, Denmark, and Kozel to Frankfurt, Germany. She said the pair booked the tickets through China Southern Airlines.

As holders of EU passports with onward flights to Europe, the passengers would not have needed visas for China.

The Thai national police chief on Monday set up a task force to investigate the issue of the stolen passports.

Interpol said it and national investigators were working to determine the true identities of those who used the stolen passports to board the flight. White House Deputy National Security Adviser Tony Blinken said the U.S. was looking into the stolen passports, but that investigators had reached no conclusions.

Interpol has long sounded the alarm that growing international travel has underpinned a new market for identity theft: Bogus passports are mostly used by illegal immigrants, but also pretty much anyone looking to travel unnoticed such as drug runners or terrorists. More than 1 billion times last year, travelers boarded planes without their passports being checked against Interpol’s database of 40 million stolen or lost travel documents, the police agency said.

Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over a large area. If the plane enters the water before breaking up, there can be relatively little debris.

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